Modern base stations typically have to support multiple standards such as GSM, WCDMA, and LTE. These standards operate in different frequency bands. The frequencies used by each standard also depend on geographical location. Moreover, in order to support the ever-increasing need for bandwidth, operators often use carrier aggregation to provide higher data rates. Unfortunately, re-designing radios for every band of operation is costly for equipment vendors. What is needed is a multi-band radio and/or multi-standard radio that is frequency agile and that avoid analog processing as much as practical.
Recently, the semiconductor industry has started to produce analog-to-digital converters (ADCs) with sampling rates in the GHz range, see for example the Texas Instrument, ADC12D1800 datasheet described at http://www.national.com/pf/DC/ADC12D1800.html#Overview. That means that the analog-to-digital boundary in a radio receiver may be moved closer to the antenna so that most if not all of the receiver signal processing may be done in the digital domain.
But analog-to-digital converters with sampling rates in the GHz range impose severe processing requirements for the digital hardware in such a digital radio receiver (sometimes referred to as a direct-RF receiver). Currently, ASICs and FPGAs typically have operating speeds below 500 MHz. So the implementation of a direct-RF receiver is a challenging task.